Delay feedlist invalidation until property changes are saved

This fixes part 3 of #111 (edits to 'Include in Today view' do
not refresh the unread counts immediately). The invalidation
used to occur before omitInUnread is saved to brief.sqlite
Copy Query.folders by value

Query.folders need to be copied by value so that the list
is not modified when children are added to _effectiveFolders

This fixes part 1 of #111 (headlines mode switch not working
with nested folders)